Issues with the direct impingement operation system of the AR15/M16 rifle (and variants thereof) are well known. Many attempts have been made to replace the original direct impingement operation system with a gas piston system. Some proposals are retrofit systems, in which the original gas tube is replaced with a piston and cylinder for short stroke operation of the bolt. These systems typically use the existing buffer spring that extends into the butt stock. Other solutions have been proposed in which an entirely new rifle is designed to include operator controls similar to those familiar in the AR15/M16 platform. For example, Remington Arms has produced its Adaptive Combat Rifle (ACR) and FN Herstal (FNH USA) has proposed its MK16 and MK17 rifles, the latter of which was adopted by the United States Special Operations Command (SOCOM) as a result of the Special Operations Forces Combat Assault Rifle (SCAR) competition. Each of these examples includes a recoil spring within the upper receiver, allowing the butt stock to be folded, if desired. These designs, however, utilize a unique (nonstandard) lower receiver and require complete replacement of existing rifle systems, rather than allowing modification of existing weapons currently in inventory.